Theories and concepts largely influence the manner by which investigators will view or understand his or her surroundings (Wiley, 1978). More specifically, in scientific investigations, it is very crucial that an investigator or a researcher clings to reputable and empirically-concluded theories in order to come up with appropriate observations and research based conclusions (Wiley, 1978). The same concept holds true for the Evolutionary Biology wherein researchers are troubled by the formulation and recognition of the most applicable and most appropriate species concept. Such kind of conceptual framework is very essential in the said scientific field because it gives clarification on the rightful manner by which the species, the last entry on the classification of the hierarchy of biological units (Wiley, 1978), should be considered. Hence, the objective of this essay is to describe the various species by providing emphasis on two specific conceptual frameworks, phylogenetic and non-phylogenetic, in order to provide detailed information on the currently available species concepts which are recognized by the scientific community. The evolutionary-based and non-evolutionary based concepts are chosen because they are the ones which are seen to render the most number of scientific evidences from the literature. Furthermore, the aforementioned concepts will provide a sound basis of the perception of a person on his or her importance in the biosphere.
The Evolutionary Species Concept was originally formulated by Simpson in 1961 and it states that species is a single lineage of ancestral descendant populations of organisms which maintains its identity from other such lineages and which has its own evolutionary tendencies and historical fates (as cited by Wiley, 1978 p. 18). By analysis, this definition warrants the perception of species as individuals rather than classes and as historical, temporal and spatial entities (Wiley, 1978). Consequently, the Evolutionary Species Concept is said to be more valid than the other concepts because it takes into account all of the organisms of evolutionary relationships, effectiveness of reproductive isolation to allow maintenance or species identity, non-consideration of morphological distinctiveness, and the notion that no single lineage can produce ancestral-dependent species (Wiley, 1978).
The abovementioned species concept is based on the phylogenetic relationships of species and in contrast to this, biologists may opt to define species based on non-phylogenetic considerations (Agapow et al., 2004) which include The Biological Species Concept, The Taxonomists Species, and The Nominal Species (Lewin, 1981). The Biological species concept regard species as group of organisms that have no capability of interbreeding thereby saying that species are reproductively isolated from each other, a representation of separate biological lineages (Agapow et al., 2004). This theoretical framework considers self-recognition and interfertility as determined by conspecificity, gene reassortment, gene flow, genetic divergence, sexual maturity and other fertility-based concepts (Lewin, 1981). More specifically, it states that members of the same species are capable of interbreeding while members of non-adjacent species are incapable of mating as dictated by certain biological and genetic barriers (Lewin, 1981). In a less scientific manner, the BSC describes the recognition of species in the way by which the organisms perceive it themselves (Lewin, 1981). The Taxonomists species concept, on the other hand, states that a species is a class of organisms in most respects, differing only by the kind of individual variation that can be experimentally shown to be attributable to environmental factors or to freely segregating and recombining genes (Lewin, 1981, p. 612). This definition reflects the way by which investigators perceive species based on the extending field of experience and techniques. Lastly, the Nominal Species Concept affirms the importance of the usage of binomial Latin names in the distinction of organisms and such naming system provides a logical and internationally agreed upon manner of naming species in order to facilitate research studies and interactive learning (Lewin, 1981).
Mechanisms that make speciation possible is composed of three factors and these are allopatric, parapatric, and sypmpatric (Species concepts, phylogenies, and speciation). Allopatric speciation explains that the new species evolves as a function of geographical isolation from its ancestors while the parapatric speciation states that the new species evolve in a geographical contiguous population (Species concepts, phylogenies, and speciation). Lastly, sympatric speciation is characterized by the evolution of species within the geographic range of its ancestors (Species concepts, phylogenies, and speciation).

Researchers believe that species concept should be logically distinct from particular mechanisms of speciation (Chandler Gromko, 1989). It is also emphasized that different variables were significantly associated with species richness in each arrangement of taxonomy (Isaac Purvist, 2004). Hence, the notion that species are passive end products of evolution, which is based upon the low level of intraspecific gene flow and divergent populations when exposed to uniform selection pressures, is improper and must not be accepted because such considerations of traditionally perceived constraints of genetic variations are overemphasized and are less likely to cause significant species differentiation (Rieseberg Burke, 2001). Species should be considered as active aggregates of the biological process which are very essential in the completion of ecological cycles. Mechanisms of speciation are important but the notion of species concept should be grounded on the fusion of both historical evidences and present conditions of the organisms. In this manner, both the phylogenetic and non-phylogenetic means of species concept are acceptable but are subjected to subsequent scientific analysis.

The phylogenetic theory is essential in the analysis of fossil-based evidences while the non-phylogenetic frameworks are needed in the study of present species status with respect to the whole biological system and other non-living organisms. The concepts laid in this essay stress the importance of knowing and establishing a definition of the species concept. However, the same process suggests to the readers that a single conceptual framework is not yet applicable for all the species across all cultures and covering all geographical boundaries. Consequently, presently available species concept all affirm to the idea that all species are essential in the processes of life on earth and no organism must be considered inferior to the other because all are products of complicated evolutionary processes and species differentiation.